Myocardiogenesis will be studied in normal and cardiac lethal mutant axolotls, Ambystoma mexicanum, and in normal and cardiomyopathic Syrian hamsters, strain UM-X 7.1. Morphological, immunohistochemical, biochemical, and tissue culture methods will be employed. The primary objectives of the studies are to elucidate the sequence of events and mechanisms of myofibrillogenesis and to determine how inductive interactions operate to control heart differentiation. The morphological studies will be used to analyze structural abnormalities in early embryos of the axolotl and hamster heart systems in order to better understand the onset of heart function. Immunohistochemical methods at both light and electron microscopic levels will be used to localize within the cells the sites of synthesis and subsequent relocations of the various contractile proteins as they assemble to form myofibrils. Isoelectric focusing, SDS-polyacrylamide gel electrophoresis and radioimmunoassays will permit detection and quantitation of the contractile proteins as embryonic heart development progresses. Tissue culture of heart cells in well-defined environments will permit an evaluation of the primary defects resulting from the mutant genes. In addition, cultured cells microinjected with fluorescently-tagged proteins will provide insight into the dynamic aspects of myofibrillogenesis. Heart induction in axolotls will be studied by organ culture and biochemical methods. An attempt will be made to isolate, characterize, and understand the mechanisms of the inductive substance(s) responsible for turning noncontractile mutant hearts into vigorously contracting normal ones. The use of these two unique genetic animal models, both which result in abnormal heart development, will hopefully provide the "tools" neceseary for determining what is required for normal myofibrillogenesis to take place and for the initiation of normal heart function to begin. In a broader sense, these vertebrate birth defects are potentially capable of providing an answer to one of the major unsolved problems of modern biology: the control of gene expression in animals.